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The data on the hillslope and channelized debris flows in the Shitou area of central Taiwan occurred during Typhoons Toraji and Nali in 2001 were applied in this paper. The geomorphic parameters, including the flow length, gully gradient, drainage area and form factor of the debris flows were determined by spatial analysis using a Geographic Information System (GIS) based on the data derived from field investigation, aerial photographs, and topographical maps. According to such determined geomorphic parameters, the threshold conditions and empirical equations, such as the relationship between the gully gradient and drainage area and that between gully length and drainage area and topographic parameter, are presented and used to distinguish the geomorphic characteristics between the channelized and hillslope debris flows.

A probabilistic equation of design discharge for a mountainous gully was derived in terms of reliability probability. Its derivation was based on the rational formula, the rainfall-intensity-duration formula, and the First-Order Second-Moment method (FOSM) by considering the uncertainty of parameters, such as the annual rainfall, the runoff coefficient, the watershed area, the sediment concentration, the length and velocity of overland flow, and the horizontal length and elevation difference of a gully, involved in the formulas. The probabilistic equation was also applied to the probabilistic design discharge for a debris-flow-prone gully in Taiwan. RÉSUMÉUne équation probabiliste de débits de projet pour un ravin en montagne a été établie en termes de fiabilité. Son établissement a été basée sur la formule rationnelle, la formule de précipitation-intensité-durée, et la méthode au premier ordre du moment du second ordre (FOSM) en considérant l'incertitude des paramètres, tels que les précipitations annuelles, le coefficient d'écoulement, la surface de précipitation, la concentration en sédiment, la distance et la vitesse de l'écoulement de surface, la longueur horizontale et les différences de cotes du ravin, impliqués dans les formules. L'équation probabiliste a été également appliquée au débit probable de projet pour un caniveau d'écoulement de débris à Taiwan.

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